Cystic fibrosis transmembrane conductance regulator (CFTR) functions as an anion channel, and is essential for fluid and electrolyte homeostasis at epithelial surfaces of many organs, including lung and intestine. The autosomal-recessive disorder cystic fibrosis (CF) is caused by mutations of the CFTR gene. CF disease is highly variable, and patients have a median life expectancy of approximately 40 years. Loss-of-function mutations cause altered ion and fluid transport, which results in accumulation of viscous mucus in the pulmonary and gastrointestinal tract. This is associated with bacterial infections, aberrant inflammation and malnutrition. Over 1500 mutations have been described, but the most dominant mutation (˜67% of total mutant alleles world-wide) is a deletion of phenylalanine at position 508 (CFTR-delF508). This causes misfolding, ER-retention and early degradation of the CFTR protein which prevents function at the plasma membrane. Other mutations in the CFTR gene that have been found in CF patients also impair protein folding or impair protein production, gating, conductance, splicing and/or interactions with other proteins.
Current therapy for CF is mainly symptomatic and focuses on reduction of bacterial pressure, inflammation, and normalization of nutrient uptake and physical growth. Recently, multiple compounds have been identified that target mutation-specific defects of the CFTR protein itself. Clinical trials are currently performed using compounds that induce i) premature stop codon readthrough, ii) correction of plasma membrane trafficking of CFTR (correctors), and iii) enhance CFTR gating (potentiators). Recently, a phase III clinical trial has successfully been completed for a potentiator in CF patients with a CFTR-G551D mutation, demonstrating that mutation-specific drug targeting is feasible in CF. Combinations of correctors and potentiators are currently assessed in a phase II trial for the dominant patient-group harboring the CFTR-delF508 mutation.
Although these recent developments are very promising, the level of functional restoration of CFTR by these drugs in in vitro model systems is still limited. In addition, patients show variable responses to these therapies due to yet undefined mechanisms. The inability to select these non-responding subgroups limits clinical efficacy and drug registration. Together, this indicates that development of new compounds and efficient screenings of drug efficacy at the level of individual patients, as well as the screening of large libraries to identify novel compounds are urgently needed. Thus far, there are no primary cell models available to screen for compounds that restore mutant CFTR function, only transformed cell lines have been used to identify compounds and their efficiency. An in vitro model which allows for the expansion and maintenance of primary human cells will allow the analysis of the drug response of individual patients and identify subgroups of responsive patients for each treatment. In addition, it will allow the screening of libraries of novel drugs for their effect on primary cells.